1. Field of the Invention
The present invention relates to an optical deflection apparatus including a microstructure, a method of driving an optical deflection apparatus, and an image forming apparatus including an optical deflection apparatus, such as a laser beam printer.
2. Description of the Related Art
Recently, optical deflection apparatuses that perform scanning with light beams are used in image forming apparatuses such as optical disc readers, barcode readers, laser beam printers, and digital copying machines.
As a type of such optical deflection apparatuses, resonant optical deflection apparatuses have been proposed, in which typically micromirrors formed by silicon micromachining techniques are driven by resonance.
A resonant optical deflection apparatus can be constructed in a much smaller size when compared with an optical deflection apparatus in which a rotary multiple-face mirror such as a polygon mirror is used. Furthermore, the resonant optical deflection apparatus is advantageous in that power consumption is small, face tangle does not occur theoretically, and in the case of an optical deflection apparatus formed of Si monocrystal, manufactured by semiconductor processes, metal fatigue does not occur theoretically so that durability is excellent.
According to techniques described in U.S. Pat. No. 4,859,846, driving based on a triangular wave is achieved by using an optical deflector having a vibration mode at a fundamental frequency and a vibration mode at a frequency that is triple the fundamental frequency. FIG. 7 shows a micromirror that is driven according to a triangular wave. An optical deflection apparatus 12 includes oscillators 14 and 16, torsion springs 18 and 20, drivers 23 and 50, detectors 15 and 32, and a control circuit 30. This micromirror has a fundamental resonant frequency and a resonant frequency that is substantially triple the fundamental resonant frequency, and is driven according to a combined frequency of the fundamental resonant frequency and the substantially triple resonant frequency. Thus, the oscillator 14 having a mirror surface is driven according to a triangular wave, so that the deflection angle has less change in the angular velocity compared with a case of driving according to a sine wave. At this time, vibration of the oscillator 14 is detected by the detectors 15 and 32, the control circuit 30 generates a driving signal needed to generate a triangular wave, and drivers 23 and 50 drive the micromirror.
Furthermore, International Publication No. WO2005/063613 discloses a micro oscillator in which a system including a plurality of torsion springs and a plurality of movable members have a plurality of separate natural vibration modes. In the micro oscillator, the plurality of separate natural vibration modes includes a reference vibration mode, which is a natural vibration mode at a reference frequency, and an even-multiple vibration mode, which is a natural vibration mode at frequency that is substantially an even-integer multiple of the reference frequency. According to International Publication No. WO2005/063613, for example, driving according to a sawtooth wave is achieved by causing the micro oscillator to vibrate in these oscillation modes.
Furthermore, according to Japanese Patent Laid-Open No. 2005-292627, in order to detect a scanning position of a light beam deflected by a deflection mirror driven according to a sine wave, a time at which the deflected scanning light beam passes through a predetermined point is detected by an optical sensor, and the deflection mirror is controlled on the basis of the detected time.
The resonant frequency of a resonant optical deflector is determined according to the mass of an oscillator and the spring constant of a torsion spring in the optical deflector. Since the spring constant of the torsion spring changes depending on environmental factors such as temperature, the resonant frequency of the resonant optical deflector also changes depending on environmental factors.
In order to correct such a variation in the resonant frequency of the optical deflector, for example, according to techniques described in Japanese Patent Laid-Open No. 9-197334, a heat generator is provided in an optical deflector to heat a torsion spring and thereby adjust the resonant frequency.
Although it is possible to drive an oscillator in an optical deflection apparatus according to a triangular wave or a sawtooth wave according to the documents described above, further improvement is desired regarding control of the deflection angle of the oscillator.
Particularly, control of the deflection angle is an important issue in a case where the resonant frequency of an optical deflector changes due to environmental factors.